1. Field of the Invention
The present invention relates to a CDMA system and, more particularly, to a base station transmitter in a CDMA system.
2. Description of the Related Art
FIG. 1 is a block diagram of a conventional base station transmitter in a CDMA system. Referring to FIG. 1, the conventional base station transmitter in a CDMA system includes a plurality of base station modems 100 for direct-spectrum-spreading PCM data, a digital combiner 110 for summing up CDMA signals outputted from the base station modems 100 by channels (I, Q), an IF processor 120 for converting base-band CDMA signals outputted from the digital combiner 110 into intermediate frequency (IF) signals, and an RF processor 130 for converting the IF CDMA signals into radio frequency (RF) signals, amplifying the RF signals and sending them to an antenna.
The base station modems 100 includes a Walsh generator 101 for multiplying a CDMA signal by Walsh code, first and second multipliers 102 and 103 for multiplying the CDMA signals outputted from the Walsh generator 101 by PN (Pseudo Noise) codes PN-1 and PN-Q1, respectively, to direct-spectrum-spread the CDMA signals, and first and second finite impulse response (FIR) filters 104 and 105 for filtering the spectrum-spread CDMA signals. The digital combiner 110 consists of first and second adders 111 and 112 for adding up the CDMA signals spectrum-spread by the digital combiner 110 by sectors. Here, the CDMA signals are transmitted in 14-bit serial bit streams.
The base band/IF signal processor 120 includes first and second serial-to-parallel converters (not shown) for converting the digital CDMA signals into parallel signals, first and second D/A converters 121 and 122 for converting the parallel CDMA signals into analog CDMA signals, first and second phase equalizers/low pass filters 123 and 124 for compensating the phases of the analog CDMA signals and removing unnecessary signal components from the signals, first and second mixers 125 and 126 for multiplying the CDMA signals whose phases were compensated by carrier signals to convert them into IF CDMA signals, and a third adder 127 for adding up the IF CDMA signals and QPSK(quadrature phase shift keying)-modulating the signal. Here, the carrier signals are COS(2πfIFt) and SIN(2πfIFt).
The RF processor 130 includes a third mixer 131 for mixing the base band CDMA signal QPSK-modulated by the base band/IF signal processor 120 with a local oscillation signal SPRF, to convert it into an RF CDMA signal, a band pass filter 133 for eliminating the spurious component of the RF CDMA signal, and an amplifier 134 for amplifying the output of the band pass filter 133 and sending it to the antenna.
The operation of the conventional base station transmitter of a CDMA system constructed as above is described below with reference to the attached drawing.
When PCM data having a predetermined data rate is inputted from a vocoder (not shown), the CDMA base station 100 direct-spectrum-spreads the PCM data into digital base band CDMA signals of 1.2288 Mcps by sectors and sends them to the digital combiner 110 through I-channel (In-phase channel) and Q-channel (Quadrature channel). This operation is respectively performed for the plurality of CDMA base stations.
The digital combiner 110 sums up the digital base-band CDMA signals outputted from the base station modem 100 by the sectors. That is, the digital combiner 110 sums up CDMA signals received from the plurality of base station modems 100 by the sectors, and then sends them to the IF processor 120 in the form of serial bit stream. The IF processor 120 converts the digital CDMA signals received from the digital combiner 110 into IF CDMA signals, respectively sums up the IF CDMA signals of the I-channel and Q-channel, and QPSK-modulates them.
Specifically, the first and second serial-to-parallel converters of the IF processor 120 respectively convert the base-band digital CDMA signals respectively received through the I-channel and Q-channel into parallel CDMA signals, and the first and second D/A converters 121 and 122 thereof respectively convert the parallel CDMA signals into analog CDMA signals. In addition, the first and second phase equalizers/low pass filters 123 and 124 compensate the phases of the analog CDMA signals and remove unnecessary components from the signals, and the first and second mixers 125 and 126 mix the analog CDMA signals whose phases are compensated with carrier signals to convert the base-band CDMA signals into IF CDMA signals. The third adder 127 adds up the analog IF CDMA signals received from the first and second mixers 125 and 126 to output the QPSK-modulated CDMA signal.
The third mixer 131 of the RF processor 130 mixes the QPSK-modulated CDMA signal with the local oscillation signal LORF to convert it into an analog RF CDMA signal, and the band pass filter 133 removes the spurious component from the RF CDMA signal, and then sends it through the amplifier 134.
However, the conventional base station transmitter has the following problems.
First of all, there is a high possibility that noise components may be added on the analog path following the first and second D/A converters because the IF processor converts the digital CDMA signals sent from the CDMA modem into the analog CDMA signals. This affects the forward capacity of the system seriously to result in deterioration of speech quality in case of lots of traffic. Furthermore, it is difficult to improve characteristics of the CDMA signals because of the low pass filter and band pass filter. Moreover, to improve signal characteristic through the low pass filter in the conventional base station transmitter requires linearity of phase characteristic as well as the cut-off characteristic of the filter. However, currently used analog low pass filters cannot satisfy the phase characteristic and they occupy large areas and consume lots of power.
In addition, the conventional base station transmitter converts the analog CDMA signals into IF CDMA signals, and then converts them into the RF CDMA signals. For this, a surface acoustic wave (SAW) filter for processing the RF signals is generally connected to the output port of the third adder, and the mixer must be used for converting the IF signals into the RF signals. Accordingly, the conventional base station transmitter needs additional devices for converting the IF signals into the RF signals, resulting in an increase in the manufacturing cost of the system.